Add foreign_type option for polymorphic has_one and has_many.

[ci skip] fix typo in MessageVerifier#verify docs

Adding information about 'allow_nil' option for 'numericality' [ci skip]

Add guides for `required` model generator option

Related to this PR #16062

[ci skip]

Testing Guides: adding TIP about database permission problems [ci skip]

loading fixtures to the database.

Reference #17542

Fix options overwritten by super

Remove unused "Developer" fixtures from tests

[ci skip] Fix typo

The test added in 42418cfc wasn't
actually testing anything, since the bug was with TZ aware attributes
only.

PostgreSQL for example, allows infinity as a valid value for date time
columns. The PG type has explicit handling for that case. However, time
zone conversion will end up trampling that handling. Unfortunately, we
can't call super and then convert time zones.

However, if we get back nil from `.in_time_zone`, it's something we
didn't expect so we can let the superclass handle it.

Fixes #17971

The `RecursiveCallbackDeveloper` and `ImmutableMethodDeveloper` classes
are not used anymore in tests, and neither is the `@Cancelled` variable.

To be possible to use a custom column name to save/read the polymorphic
associated type in a has_many or has_one polymorphic association, now users
can use the option :foreign_type to inform in what column the associated object
type will be saved.

`freeze` will ultimately end up freezing the `AttributeSet`, which in
turn freezes its `@attributes` hash. However, we actually insert a
special object to lazily instantiate the values of the hash on demand.
When it does need to actually instantiate all of them for iteration (the
only case is `ActiveRecord::Base#attributes`, which calls
`AttributeSet#to_h`), it will set an instance variable as a performance
optimization

Since it's just an optimization for subsequent calls, and that method
being called at all is a very uncommon case, we can just leave the ivar
alone if we're frozen, as opposed to coming up with some overly
complicated mechanism for freezing which allows us to continue to modify
ourselves.

Fixes #17960

The user is able to pass PG string literals in 4.1, and have it
converted to an array. This is also possible in 4.2, but it would remain
in string form until saving and reloading, which breaks our
`attr = save.reload.attr` contract. I think we should deprecate this in
5.0, and only allow array input from user sources. However, this
currently constitutes a breaking change to public API that did not go
through a deprecation cycle.

Also avoid using try since is_a? is faster for this case.

Make error message clearer that :on requires a symbol, not a string

fixing mailer previews for apps with globbing route

add brackets around attribute_name [ci skip]

since 'attr_name_will_change!' is not an actual method it should
be clearer that you have to insert the attribute name as in line 104

[ci skip]

Add note to Store about uniqueness validation (#17954) [skip ci]

Test coverage for ActiveJob generator

minor sentences fixes

The validation added in 5a3dc809 will
reject values for the `:on` option for after_commit and after_rollback
callbacks that are string values like `"create"`.

However, the error message says ":on conditions for after_commit and
after_rollback callbacks have to be one of create,destroy,update". That
looks like a string value *would* be valid.

This commit changes the error message to say ":on conditions for
after_commit and after_rollback callbacks have to be one of [:create,
:destroy, :update]", making it clearer that symbols are required.

Fix grammar of sentence in Reflection documentation [ci skip]

[ci skip] add `enable_extension` to PostgreSQL hstore example

Mounted Rack apps should have default named routes based on app name

[ci skip] update rails version to 3.2.21